Breadth First Traversal for a Tree, Python

I figured out Depth First Traversal for a Tree.

def _dfs(tree, res):
     if tree:
         res += [tree.key]
         _dfs(tree.left, res)
         _dfs(tree.right, res)
     return res
 

I can't seem to find a solution for Breadth First Search. Will one have to use queues or stacks?

Thanks!!

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